In the analysis of rotating machinery, it is sometimes necessary to determine the relative movement of two elements. For example, in some manufacturing processes, it is necessary that two rolls be rotating absolutely in phase with each other, independent of the speed at which they are rotating. This may be accomplished by driving one roll by a motor and driving the second roll by means of a drive train connected to the first roll. In this way whatever rotational characteristics the first roll has should be imparted through the drive train to the second roll. The drive train used, however, may contain gears, timing belts, or other power transmission elements, and any backlash, misshapen teeth, or mechanical damage in such gears and belts will cause the rotational characteristics of the driven roll to vary considerably from theoretical coordination. If the frequency of the phase differences between the driving and the driven roll can be determined, the source of the phase errors can be pinpointed, since the frequency of the phase error will correspond to some characteristic frequency of the element causing it. By "frequency" here is meant the frequency with respect to the driving roll, not with respect to time. The necessary information can be obtained using digital pulse encoders coupled to the shafts of the two rolls. Each of these encoders emits a series of electrical pulses, each pulse corresponding precisely to a definite amount of shaft rotation. By comparing the number of pulses output by the two encoders, it is possible to compare the amounts of rotation of the two shafts.
In other situations, it may be necessary to determine rotational characteristics with respect to time. In these cases, the output of a digital pulse encoder coupled to the rotating element of interest can be compared to a pulse train emitted by an oscillating crystal or other generator of a known, fixed frequency.
Thus, the reference pulse train can be generated by the drive source for the driven mechanism whose characteristic is being simultaneously measured, or it can be generated by a fixed frequency oscillator, depending upon whether the measured characteristic is to be compared to the driving source or to time.
The function of the invention system is to count the two pulse trains simultaneously and record or display the number of pulses received from the caracteristic pulse train for each n pulses received from the reference pulse train, where n is a preselected number chosen to give the required resolution, the value of which is approximately dependent upon the expected frequencies of the vibration or repetitive error sought. This information provides a direct measure of the magnitude of phase errors in the system and, in addition, can be used in a variety of mathematical analyses which provide information on the frequencies of the errors.
A typical conventional apparatus consists primarily of two counters, digital control logic, and means for recording or displaying data received from the counters. Each pulse train constitutes an imput to a counter. The counters are connected through digital logic such that, when the number in the counter associated with the reference pulse generator reaches a preset value, a transfer pulse is generated and the number stored in the second (or measurement) counter is immediately transferred to storage or display means, after which both counters are reset to zero to begin another cycle. Conventional apparatus of this type is usually accurate to within one pulse per each n pulse total reading, the inaccuracy occurring whenever a transfer signal is generated at the same instant that a pulse is entering the second counter. When this happens, the number presently stored in the counter is transferred, the counter is reset, and the pulse then entering the counter is dropped, the next pulse generated being counted as the first pulse in a new cycle. For many applications this accuracy is quite sufficient. However, as hereinafter explained, there are times when it is entirely unacceptable for this "pulse dropping" to exist.
For example, it is sometimes desirable to be able to take data which can be used to detect errors over a relatively wide range of frequencies. Such a situation would exist where a drive train includes a timing belt which is long compared to the distance from one gear tooth to the next. For a complete analysis, it is necessary to have data capable of revealing discrepancies in the tooth-to-tooth spacing as well as cyclic errors which repeat only once or twice in a single travel cycle of the belt. Thus, both high and low frequency errors must be discerned. It is obvious that the conventional apparatus could take data which would be useful for detecting tooth-to-tooth discrepancies simply by choosing n, the number of reference pulses per reading, corresponding exactly with the amount of reference shaft rotation, to be small enough to take a reading each time a tooth passes a given point. Similarly, n might be chosen to be large enough to take one or two readings per travel cycle of the belt.
If, instead of taking data at this low frequency, that is, once or twice in each revolution of the belt, one attempts to utilize the higher frequency data to analyze low frequency errors, the interval of interest spans not one or two dropped pulses but many, many times that number. This can lead to entirely unacceptable levels of inaccuracy, especially where the investigator is interested in absolute positions of the shaft being measured. For this reason, it is necessary to take data at each frequency which yields meaningful results. This can be burdensome when, for instance, there are a number of components in a drive train and all are possible sources of error.
It is an object of this invention to provide an apparatus which is capable of comparing two electrical pulse series without troublesome pulse dropping.
It is another object of this invention to provide improved apparatus which is capable of taking data useful for simultaneous high and low frequency error detection and analysis.
It is a further object of the invention to provide an improved apparatus capable of taking data useful for detecting absolute velocity variations.